It has been heretofore proposed to detect the presence or absence of an electrically-conductive object in a given zone.
Thus, the so-called "eddy current killed oscillator" systems of Micro Switch of Freeport, Ill., a division of Honeywell, Incorporated, have been so used. In particular, I have found that such system, including Micro Switch's FMA2 "All Metals" (Extended Sensitivity) Amplifier, and Micro Switch's FMSAL 5-6 sensor loop is well suited, all things being equal, to detecting a strip of thin aluminum foil, an inch or so wide and about the diameter of the loop, when such strip is within about 2 inches of the loop for as short a time as 0.04 seconds.
The FMA2 is, in effect, a generator of 500 KHz alternating current which is coupled by the sensor loop into the zone in which detection is to occur. Detection may be considered to occur due to the change in loading of the generator, such change being due to sufficient eddy current induction in a foil strip, when the strip gets close enough to the loop, at least momentarily. However, the sensitivity of the system is critical, in that if it is set too low it will not detect the presence of a strip, whereas if it is too high, the generator "locks up", responding by turning off (as is intended) in response to the presence of a strip, but, when that strip departs from the detection zone remains off, (which is not intended). Again, at maximum sensitivity, inductor temperature increase alone may cause the generator to cease oscillating. While it is possible to adjust the sensitivity of detection to an intermediate value wherein the generation of 500 KHz only ceases while the foil strip is in the detection zone, the adjustment is unstable. Thus, changes in the temperature of the loop (and probably drift in circuit component values) throws the adjustment off.